This invention relates to methods and apparatus for determining the characteristics of materials surrounding a well bore, and more particularly, relates to methods and apparatus for radio frequency dielectric induction well logging wherein the formation resistivity, permittivity, water saturation and residual oil saturation are determinable by a means of in situ measurements taken in a well borehole.
For many years it has been conventional practice to log the electrical characteristics of earth formations in the vicinity of a well borehole in order to determine the location of oil bearing strata. This has been made possible in the past by the use of electrical resistivity logs in wells using highly conductive (low resistivity) drilling fluid and by the use of induction well logs in wells drilled with oil base drilling muds or drilling fluids having higher resistivities. In conventional resistivity logs a current emitting electrode or electrode array arranged for focusing the emitted current is used to emit either direct current or very low frequency (such as 60 hertz) alternating currents into the earth formation surrounding the well borehole by means of contacting electrodes. These currents traverse a section of the earth formations and are detected at a current return electrode situated a spaced distance from the current emitting electrode. The magnitude of the detected currents can then provide an indication of the resistivity of the earth formations surrounding the well bore. In some cases current electrodes are used in conjunction with measuring potential electrodes to determine the formation resistivity.
In electrical induction logging it has been conventional practice in the past to provide a well logging sonde having a transmitting coil (or array) disposed thereon and a receiver coil (or array) at a spaced distance therefrom. Generally a high frequency alternating electric current is passed through the transmitter coil (normally at a frequency of approximately 20 kilohertz). The resulting electric fields produced from this high frequency alternating current in the earth formations surrounding the well bore are detected at the spaced receiver coil by sensing the induced currents or voltages in the receiver coil.
In both of these types of prior art resistivity (or conductivity) logging systems for usefulness of the system arises from the fact that earth formations having pore spaces therein filled with hydrocarbon molecules exhibit a higher resistivity than those earth formations having the pore spaces therein filled with either salt water or some other conducting fluid.
Various problems have arisen in the interpretation of either conventional induction logging records or resistivity logging records of wells in areas where fresh water (relatively nonconductive such as less than 10,000 parts per million sodium chloride) are encountered. Such fresh water bearing sands or earth formations exhibit high resistivities (or low conductivities) much the same as those encountered in hydrocarbon bearing formations. In these cases it is difficult, if not impossible, to distinguish on the basis of the electric or induction well logging data alone whether an earth formation which is a prospective producing zone contains fresh water or hydrocarbon. It would therefore be very beneficial to provide a well logging system which on the basis of a single measurement of some of the physical characteristics of the earth formations in the vicinity of a well borehole could distinguish between fresh water bearing and hydrocarbon bearing earth formations. Such a system is provided in copending application Ser. No. 391,900, filed Aug. 27, 1973, which is assigned to the assignee of the present invention for a two frequency induction dielectric well log. The present invention uses the induction dielectric logging techniques of this aforementioned application but rather than operating at two different frequencies, makes measurements at two different effective depths of investigation at the same frequency. By appropriately combining porosity information and the two amplitude measurements made at the different effective depths of investigation according to the concepts of the invention, residual oil saturation and water saturation of the earth formations may be determined.